Tone control circuit

ABSTRACT

A tone control circuit which has a first signal path which passes an input signal directly and a second signal path which includes at least a filter and a gain control circuit and an adder receiving the first signal and the second signal and control means for providing a control voltage to the gain control circuit and switching means for controlling the gain control circuit to selectively control the output of the adder to produce an output signal with the desired selected frequency response.

TECHNICAL FIELD

This invention relates to a tone control circuit, which is provided witha filter circuit supplied with an audio signal, a gain control circuitsupplied with the output from the filter circuit and an operationalcircuit adding the input audio signal and the output from the gaincontrol circuit with each other to carry out tone control due to thecharacteristic of the filter circuit and the gain of the gain controlcircuit and to primarily control the gain with a DC voltage which issuitable for remote control.

BACKGROUND ART

At present, the television broadcast of a voice-multiple system ispractised and the quality of voice or sound in the television broadcastis remarkably improved. In association therewith, it is required thatthe sound field thereof are be improved. Further, in view of remotecontrol operation of a television receiver, it is also desired that thetone control can be remotely controlled with a DC voltage. Moreover,when the circuitry of the television receiver is made as an integratedcircuit, be it is necessary that the tone control circuit constructed asan integrated circuit.

In consideration of the above points, the inventors have alreadyproposed a tone control circuit which is different from the prior artattenuation type CR system and the feedback NF system.

FIG. 1 shows the above proposed tone control circuit in which an inputsignal from an input terminal 10 is supplied to an adder 20. The inputsignal from the input terminal 10 is also fed to a boost signal lineconsisting of a filter 31, an amplifier 32 and a gain control circuit 33and the output signal therefrom is fed to an add input of the adder 20.The input signal from the input terminal 10 is supplied further to a cutsignal line consisting of a filter 41, an amplifier 42 and a gaincontrol circuit 43 and the output signal therefrom is supplied to asubtract input of the adder 20. At the adder 20, the signal transmittedthrough the boost signal line is added to the input signal and thesignal transmitted through the cut signal line is subtracted from theinput signal. Thus, an output signal is derived from the adder 20 and issupplied to an output terminal 60. A voltage V_(C) derived from a volumecontrol 50 is applied to the gain control circuits 33 and 43 as the gaincontrol signals.

With the above circuit, if the filters 31 and 41 are each made or usedas a high pass filter, the boost and cut in a high frequency band areobtained, while if the filters 31 and 41 are each made as a low passfilter, the boost and cut in a low frequency band are obtained.

That is, when the control voltage V_(C) is in the range from a centervalue V_(O) to zero, the gain control circuit 33 is set such that itsgain is zero. On the contrary, the gain control circuit 43 is so setthat when the control voltage V_(C) is in the range from the centervalue V_(O) to the maximum value 2V_(O), its gain becomes zero.Accordingly, when the control voltage V_(C) is at the center valueV_(O), the output signals from the gain control circuits 33 and 43 areboth zero. Thus, at the output terminal 60 the input signal unchangedwill be obtained unchanged and, the frequency characteristic at thistime will be flat.

When the control voltage V_(C) is changed from the center value V_(O) tothe maximum value 2V_(O), it is set so that the gain of the gain controlcircuit 33 increases and becomes 1 at the maximum value 2V_(O).Accordingly, at this state, if the gain of the amplifier 32 is taken asA₁, as shown in FIG. 2 the gain of the boost signal line becomes A₁.Therefore, the boost becomes a high boost of A₁ +1 where of course A₁>0.

On the contrary, when the control voltage V_(C) is changed from thecenter value V_(O) to zero, it is set so that the gain of the gaincontrol circuit 43 increases and becomes 1 at zero. Accordingly, at thisstate, if the gain of the amplifier 42 is taken as A₂, as shown in FIG.3 the gain of the cut signal line becomes A₂. Therefore, the cut becomesa high cut of 1-A₂ where 0≦A₂ ≦1. Thus, if A₂ =1, the attenuation can bemade infinitive.

As to the boost and cut in the low frequency band, if the filters 31 and41 are each made as a low pass filter, the operation of the circuitbecomes entirely similar to that described above.

According to the above circuit, since the control curves can be freelyset both in the boost and cut directions, the characteristic which ispreferable for acoustic response can be easily obtained. Further, sincethe control 50 has no relation to the constants of the filters 31 and 41and is controlled by the control voltage V_(C), the circuit is suitableto be remotely controlled.

For simplifying the circuit, it may be desired as shown in FIG. 4 thatthe filter 31 be used in common to both of the boost and cut lines. Whenthe circuit is made as an integrated circuit, if only one filter 31 isused the number of pins for capacitors to be externally connected isreduced by one and hence the cost of the circuit can be substantiallydecreased.

However, if the filter is used in common in the cut and boost lines, theboosting frequency and cutting-off frequency do not coincide on theabscissa of frequency f as shown in FIG. 5, because the boost gain andthe cut gain are different as described above. This is not desirable forauditory response.

Accordingly, this invention has an object to provide a tone controlcircuit which can be controlled by a DC voltage, and which is suitableto be formed as an integrated circuit and which has cut-off frequencieswhich are coincident with each other between the boost and cut modes.

DISCLOSURE OF INVENTION

This invention has a fundamental structure in which a filter is commonlyused in both boost and cut modes and the signal passed through thefilter and a gain control circuit and an input signal are calculated,and in which it is discriminated based on the control voltage for thegain control circuit whether the boost mode or cut mode is used and thefilter, gain control circuit or amplifier is changed over in response tothe discriminated output. Since in this invention the control is carriedout by the DC voltage, this invention is suitable for remote control.Further, since the filter is used commonly, the circuit construction canbe simplified and the number of pins for external connection can befewer when the circuit is made as an integrated circuit, the controlcurve can be freely set and the cut-off frequency will not deviatebetween the boost and cut lines.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a systematic diagram of a tone control circuit for theexplanation of this invention;

FIGS. 2 and 3 are graphs showing its characteristics;

FIG. 4 is a systematic diagram of a tone control circuit used to explainthis invention;

FIG. 5 is a graph showing the characteristic thereof;

FIG. 6 is a systematic diagram showing an example of the tone controlcircuit according to this invention;

FIG. 7 is systematic diagram illustrating another example of the tonecontrol circuit according to this invention; and

FIG. 8 is a connection diagram depicting an example of the tone controlcircuit according to this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In order to explain this invention in more detail, this will beexplained with reference to FIGS. 6, 7 and 8.

FIG. 6 shows an example of the tone control circuit according to thisinvention, in which the control voltage V_(C) from the volume 50 is fedto a change-over circuit 70 to provide a change-over signals S_(C) whichhave different states for the boost mode and the cut mode. When thevoltage V_(C) is higher than the center value V_(O) as set forth above,the circuit is in the boost mode, while when the voltage V_(C) is lowerthan the center value V_(O), the circuit is in the cut mode. Therefore,the change over circuit 70 is constructed such that it compares thecontrol voltage V_(C) with the reference voltage V_(O). The change-oversignal S_(c) is supplied to the filter 31 to change the time constant ofthe filter 31 between the boost and cut times. As will be apparent fromFIG. 5, in the case of the treble it is sufficient that the timeconstant in the boost mode be selected to be smaller than that in thecut mode, while in the case of the base it is sufficient that the timeconstant in the boost mode be selected to be larger than that in the cutmode.

According to the tone control circuit of this invention, as is shown inFIG. 5 the characteristics in the boost and cut modes become such thatthe cut-off frequency is equivalently shifted, and hence thecharacteristics become favourable for auditory response.

FIG. 7 shows another example of this invention. In this example, it maybe possible that the gain control circuit 33 is used in common to theboost and cut modes and serves as the amplifier and is changed over bythe change-over signal S_(C) in both boost and cut modes.

That is, in the case of the treble control in the boost mode, the gainof the circuit 33 becomes A₁ which is the gain of the above amplifier32, while in the cut mode the gain of the circuit 33 becomes A₂ which isthe gain of the above amplifier 42 and also the output becomes oppositein polarity. Since this change-over is carried out when the controlvoltage V_(C) is near the center value V_(O) and the input signal itselfis dominant, sharp variations are not caused.

FIG. 8 shows a practical example of this invention and corresponds tothe case of the treble control. An input signal from a signal source 11is supplied, to a transistor Q₀. The block for the high boost is formedof transistors Q₁ to Q₈, while the block for the high cut is formed oftransistors Q₁₁ to Q₁₈. The change-over circuit 70 is constructed oftransistors Q₂₁ and Q₂₂.

In the high boost mode where the control voltage V_(C) is larger thanthe center value V_(O), the transistor Q₂₁ is made ON but the transistorQ₂₂ is made OFF. Thus, the transistor Q₂₀ becomes ON so that a filterhaving a small time constant is formed by a capacitor C and a resistorR₁. At this time, the transistors Q₇ and Q₁₈ are in ON-state andtransistors Q₈ and Q₁₇ are in OFF-state, so that a current flows throughthe transistor Q₆ but no current flows through the transistor Q₁₆. And,while the input signal from the signal source 11 is applied unmodifiedto the base of the transistor Q₁, this input signal is supplied to thebase of the transistor Q₂ through the low pass filter consisting of thecapacitor C and resistor R₁. Thus, the signal low frequency componentsare cancelled and the signal high frequency components flow to thetransistors Q₁ and Q₂. The signal currents are derived through thetransistor Q₆ and the high frequency components are added. In this case,the gain is determined by a resistor R₃ at the emitter side of thetransistor Q₇.

While, in the high cut mode where the control voltage V_(C) is lowerthan the center value V_(O), the transistor Q₂₁ is in the OFF-state andthe transistor Q₂₂ is in the ON-state. Then transistor Q₂₀ turns OFF sothat a filter having a large time constant is formed by the capacitor Cand resistors R₁ and R₂. At this time, the transistors Q₈ and Q₁₇ areturned ON and the transistors Q₇ and Q₁₈ are turned OFF so that thecurrent flows through the transistor Q₁₆ but no current flows throughthe transistor Q₆. The input signal from the signal source 11 is fed,unmodified, to the base of the transistor Q₁₂, and the same signal issupplied through the low pass filter which is formed of the capacitor Cand resistors R₁ and R₂ to the base of the transistor Q₁₁. Therefore,the high frequency signal components flow to the transistors Q₁₂ andQ₁₁. The signal current is then derived through the transistor Q₁₆ andthe high frequency components are subtracted. In this case, the gain isdetermined by a resistor R₄ at the emitter side of the transistor Q₁₇.

The function of the transistor Q₀ is inverting and the transistors Q₃,Q₄, Q₁₃ and Q₁₄ are loads.

The addition and subtraction are performed as follows:

In the case of high boost mode, the low frequency signal which haspassed through the low pass filter comprising R₁, R₂, C and Q₂₀ isprovided to Q₂ while the input signal itself having both high and lowfrequency component is provided to Q₁. Then, due to the characteristicof the differential amplifier, the frequency components supplied to Q₁and Q₂ are cancelled and the high frequency signal having the same phaseas the input signal appears at the collector of Q₂. Then, this highfrequency signal is inverted by Q₆ and has the same phase as the inputsignal inverted by Q₀. That is, the high frequency component is added tothe input signal at point 60. This is how the addition is performed.

On the other hand, in the case of the high cut mode, the low frequencycomponent is also cancelled by the differential amplifier of Q₁₁ and Q₁₂in the same manner as above, and the inverted high frequency componentappears at the collector of Q₁₂. Then, this signal is inverted by Q₁₆and has the reverse phase of the input signal inverted by Q₀.Accordingly, high frequency component is subtracted from the inputsignal at the point of 60. This is how the subtraction is performed.

The basic circuit of the gain control amplifier shown in FIG. 8 iswell-known in the art. For example, a similar circuit is shown in U.S.Pat. No. 3,676,789.

We claim:
 1. A tone control circuit having a first signal path passing an input signal directly and a second signal path having at least a filter and a gain control circuit, comprising:(a) calculating means for adding a signal from said first signal path to a signal from said second signal path to form an output signal; (b) control means for providing a control voltage for said gain control circuit; and (c) switching means for determining a frequency mode of said output signal from said control voltage and changing the frequency characteristic of said second signal path in accordance with said frequency mode.
 2. A tone control circuit according to claim 1 wherein the frequency characteristic of said filter is switched in accordance with said frequency mode.
 3. A tone control circuit according to claim 1 wherein the frequency characteristic of said gain control circuit is switched in accordance with said frequency mode.
 4. A tone control circuit according to claim 1 wherein said control voltage is a DC voltage.
 5. A tone control circuit according to claim 2 wherein the time constant of said filter is switched in accordance with said frequency mode.
 6. A tone control circuit according to claim 1 wherein said frequency mode is a boost or a cut of a certain frequency band. 